To overcome the previously mentioned limitations of dictionary based approaches, efforts were made to handcraft a set of rules to alleviate the reliance on the completeness of dictionaries. Rules can be created to expand the dictionaries to identify previously unseen mentions. For example, MetaMap [1]}, [2]} makes use of external knowledge sources of biomedical terms—the SPECIALIST lexicon, and it employs complex rules to identify all possible mention variants of an entity, including acronyms, abbreviations, synonyms, or derivational variants. Table REF  is an example that illustrates how expansion rules are used to generate variants given a word. Expansion rules include `i' (inflection), `p' (spelling variant), `a' (acronym/abbreviation), `e' (expansion of acronym/abbreviation), `s' (synonym) and `d' (derivational variant). For example, the expansion rule of variant `ophthalmia'—`ssd'—indicates that it is a derivational variant of a synonym (`ophthalmic') of a synonym (`eye') of `ocular'.
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